The term overweight is generally used to indicate that a human (or other mammal) has more body fat than is considered useful for the optimal functioning of the body. Being overweight is a fairly common condition for many people, especially those in developed nations where food supplies are plentiful and lifestyles often do not involve a lot of activities that generate caloric expenditure. Recent studies have indicated that as much as 64% of the adult US population is overweight, and this number is increasing.[1] A series of graphics from the CDC also describes the obesity prevalence trends in the U.S. in the past 2 decades: Obesity Epidemic: U.S. Temporal Trends 1985-2004

A healthy body requires a minimum amount of fat for the proper functioning of the hormonal, reproductive, and immune systems, as thermal insulation, as shock absorption for sensitive areas, and as excess energy for future use. But the accumulation of too much storage fat can impair movement and flexibility, and can alter the appearance of the body in ways that some may find undesirable or physically unattractive.

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The degree to which a person is overweight is generally described using an indication of the amount of excess body fat present. There are several common ways to measure the amount of fat present in an individual's body.(See also body fat percentage):

Simple Weighing: The weight of the individual is measured and compared to an estimated ideal weight. This is the easiest and most common method, but by far the least accurate, as it only measures one quantity (weight) and often does not take into account many factors such as height, body type, and relative amount of muscle mass.

Body Mass Index (BMI): This is an adaptation of simple weighing which attempts to take into account the subject's general body size by dividing the weight by the height squared (the units for BMI are kg/m2, but are rarely referenced, and BMI numbers are typically written and used as unitless numbers). This provides a slightly more accurate representation than simply measuring raw weight, but still ignores many factors which can affect the results, and is generally not accurate for many individuals.

Skinfold Calipers or "pinch test": With this method, the skin at several specific points on the body is pinched and the thickness of the resulting fold is measured. This measures the thickness of the layers of fat located under the skin, from which a general measurement of total amount of fat in the body is calculated. This method can be reasonably accurate for many people, but it does assume particular patterns for fat distribution over the body which may not apply to all individuals, and does not account for fat deposits which may not be directly under the skin. Also, as the measurement and analysis generally involves a high degree of practice and interpretation, for an accurate result it must be performed by a professional and cannot generally be done by patients themselves.

Bioelectrical impedance analysis: This method involves passing a small electrical current through the body and measuring the body's resistance to the electrical flow. As fat and muscle conduct electricity differently, this method can provide a direct measurement of the percentage of body fat present as compared to muscle mass. In the past, this technique could only be performed reliably by trained professionals with specialized equipment, but it is now possible to buy "home kits" which allow individuals to do this themselves with a minimum of training. Despite the improved simplicity of this process over the years, however, there are a number of factors which can affect the results, including hydration and body temperature, so a fair amount of care must still be taken when applying this test to ensure that the results are in fact accurate and applicable.

Hydrostatic Weighing: Considered one of the more accurate methods of measuring body fat, this technique involves completely submerging the subject underwater and using special equipment to measure his or her weight while submerged. This weight is then compared with "dry weight" as recorded outside the water to determine overall body density. As fat is less dense than muscle, careful application of this technique can provide a reasonably close estimate of fat content in the body. This technique does, however, require expensive specialized equipment and trained professionals to administer it properly.

DEXA (dual energy X-ray absorptiometry): Originally developed to measure bone density, DEXA imaging has also come to be used as a precise way to determine body fat content by using the density of various body tissues to identify which portions of the body are fat. This test is generally considered to be very accurate, but requires a great deal of expensive medical equipment and trained professionals to perform.

Despite the inherent inaccuracies, the most common method for discussing this subject used by researchers and advisory institutions is body mass index (BMI) numbers. Definitions of what is considered to be overweight change from time to time and sometimes from country to country, but the current definition proposed by both the US National Institutes of Health and the World Health Organization designates anyone with a BMI of 25 kg/m2 or more to be overweight.

BMI, however, does not account for differing amounts of muscle mass, genetic factors, or many other individual variations, and thus many individuals can have BMIs less than 25 and still be considered overweight, while others may have BMIs significantly higher without falling into this category[2]. Many of the more accurate methods mentioned above for determining body fat content can provide better indications of whether a particular individual is overweight or not.

If an individual is sufficiently overweight that excess body fat could present substantial health risks, he or she is considered to be obese. It is possible for someone to be overweight without being obese (according to the NIH and WHO, a BMI between 25 and 30 is considered to be "overweight" but not "obese"). Again, the designation of "obesity" is subject to a great deal of interpretation and many individual factors, so an individual with a BMI well below 30 may be considered to be obese depending on their particular condition, while in some cases a BMI above 30 may not actually indicate obesity (although likely still does indicate being overweight).

While the health issues associated with obesity are well accepted within the medical community, the health implications of the overweight category are more controversial. The generally accepted view has been that overweight often shares adverse risks with obesity, relative to normal weight. Adams et al. estimate that risk of death increases by 20 to 40 percent among overweight persons.[3]

The amount of body fat is regulated to some extent subconsciously by the brain (by controlling caloric intake through appetite and food preferences). Although the exact mechanisms by which this occurs are not entirely known, one common theory suggests that each person may possess an inherent "set point" weight which the brain attempts to maintain, and that this set point may vary for each individual depending on a variety of factors including genetic predisposition, environment, and past experience.

This leads to the conclusion that some individuals may be predisposed to naturally maintaining different body weights than others, and thus it may be easier for some people to avoid being overweight, while others may find it much more difficult. It also suggests, however, that an individual's set point may be changeable with appropriate environment and conditioning.

A large number of people undergo some form of treatment to attempt to reduce their weight, usually either in an attempt to improve their health, to improve their lifestyle, or for cosmetic reasons. The generally recommended treatment for being overweight is a modified or controlled diet in conjunction with increased exercise. For those who are obese rather than overweight, more intensive therapies such as drugs or surgery are sometimes used (see Obesity).

Studies suggest that reducing calorie intake by itself (dieting) may have short-term effects but does not lead to long-term weight loss, and can often result in gaining back all of the lost weight and more in the longer term. For this reason, it is generally recommended that weight-loss diets not be attempted on their own but instead in combination with increased exercise and long-term planning and weight management.

The health benefits of weight loss are also somewhat unclear. While it is generally accepted that for significantly obese patients, losing weight can reduce health risks and improve quality of life, there is some evidence to suggest that for merely overweight patients, the health effects of attempting to lose weight may actually be more detrimental than simply remaining overweight[5]. Moreover, for all individuals, repeatedly losing weight and then gaining it back ("yo-yo dieting"), is believed to do more harm than good and can be the cause of significant additional health problems. This is caused by the loss of more muscle than fat.

There is no healthy, short-term solution for solving obesity, or being overweight. Changes in lifestyle, such as more exercise or dieting, must be permanent changes.